The invention is directed to black pigmented UV-hardening printing inks.
UV hardening printing inks have been known for some time in the patent literature (see German AS No. 2,358,948, the entire disclosure of which is hereby incorporated by reference and relied upon). Recently, however, there have been develop UV-printing inks which have a certain aging experience. However, there are associated with all previously known UV-printing ink systems a series of deficiencies which in part are severe so that they could not previously be placed on the market.
In contrast to conventional printing inks, UV printing inks possess several potential advantages. They can be formulated completely free from solvent. For example in web printing they are substantially more favorable to the environment than conventional heat set printing inks in which in part considerable amounts of unburned or partially burned mineral oils are emitted into the atmosphere. In sheet printing, particularly in the production of high quality multicolor prints or in printing on low or non-absorbing printing materials, as special coated papers, metal or plastic soils, the speed of working because of the low speeds of the oxidative drying systems is frequently so low that there must be allowed up to 10 hours and more from printing to the further processing such as cutting, folding, etc., as a result of which the productivity is low, the capacity of the expensive printing machines not utilized and higher storage costs arise. In contrast with the UV-hardening printing inks an immediate drying after the sheet leaves the printer is attained on any kind of printing material without set-off problems in the pile or the need to cover them with powder.
Compared to conventional printing inks UV-printing inks also have several industrial printing advantages in regard to the quality of the printing. For example they permit the production of finished prints with higher rub-resistance. Through the possibility in multicolor printing to quickly dry or dry the surface after each individual color application the quality reducing false printing "doubling" is completely prevented. The difficult and frequently protracted exact formulation of the color sequence with viscosity gradations and tack gradations of the wet in wet printing, particularly in letter press printing, is simplified and made problem free by drying each individual color before the next application of color.
In order to actually attain all of these potential advantages of UV-printing inks compared to customary printing inks a UV-printing ink must satisfy the following four main requirements.
1. Hardening Speed
An extremely high hardening activity under UV light is necessary with the smallest possible number of heavy duty --UV--lamps, e.g., a Hg--high pressure lamp with a specific output of 80 watts per cm. arc length and per color coating. The economy of the process depends strongly on the investment costs of the UV drying plant used and of the energy and maintenance costs of the drying. In offset web printing the investment costs of a UV-drying unit are considerably less than with customary heat set hot drying units. However, the running operating costs with UV-hardening can be clearly higher if too high a lamp output must be installed if the hardening activity of the UV-inks is too low. In sheet printing, in the case of paper printing, investment costs and operating costs are newly added. These costs must compete with the cost savings through higher productivity in printing with UV-inks.
2. Using up in Printing
The industrial printing properties of the UV-printing inks in regard to viscosity, tack, viscosity and tack maintenance with changes in temperature and changes in speed of printing, especially the using up in printing at very high printing speeds, in offset printing additionally behavior against water, must correspond to the state of the art conventional printing inks. In order to attain a very quick UV-hardening UV-printing inks must be formulated completely different from conventional printing inks. In order to fulfill the here described requirements with the completely differently formulated printing ink binders, i.e. UV-printing inks must be able to be used up at high speeds of printing likewise free from problems as with conventional printing inks, it is not sufficient to put together a composition of quickly polymerizing materials frequently mentioned in the patent literature under "cross-linking monomers" or "photopolymerizable monomers." Selection and combination of individual components of the binder for UV-printing inks must take place very careful not only in regard to higher hardening speed but more especially also in regard to optimum using up.
3. Toxicity, Skin Irritation, Corrosiveness against Printing Machines and Printer's Copy Material.
In order to make possible a very quick UV-hardening in the synthesis of the photopolymerizable compounds there must be built in components which are more or less toxic as well as irritating to the skin and mucous membranes and which additionally can act to dissolve or swell the roller coating in the printing machine as well to dissolve or swell the coatings of the printing plates. Therefore the components concerned in the synthesis of the binder must be so constituted in their chemical structure and in their physical properties that the negative effects of the quickly polymerizing components are held as low as possible or eliminated.
4. Storage Stability
The printing inks should have as high as possible storage stability of at least 3 to 6 months. The storage stability under the exclusion of light depends substantially upon the stability of the monomers used and the thermal stability of the photoinitiator used.
The stability of the monomers, in the event that no too great a sensitivity is present, is regulated simply by addition of customary polymerization inhibitors. A large number of the photoinitiators generally known to those skilled in the art, particularly effective photoinitiators, however, even in the dark polymerization liberating radicals which are not able to be intercepted even by large amounts of inhibitors without hindering the photopolymerization. Therefore the photoinitiators used must be carefully selected particularly for very quick drying even with regard to storage stability.
Although it is known and customary to pigment printing inks black with carbon blacks of various origins, the known types of carbon black fail to work in using printing inks hardening with UV radiation. The printing inks with known types of carbon black are inclined to thicken with storage and to harden too slowly with the UV irradiation.